Secondary batteries, which can be charged and discharged, have attracted considerable attention as power sources for devices that require high power and large capacity such as electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (Plug-in HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles that use fossil fuels.
Such devices use a middle or large-sized battery module having a number of battery cells electrically connected to provide high power and large capacity.
Since the middle or large-sized battery module is preferably manufactured to have as small a size and weight as possible, prismatic batteries or pouch-shaped batteries, which can be stacked with high integration and have a small weight to capacity ratio, are usually used as battery cells (i.e., unit cells) of middle or large-sized battery modules. In particular, much interest is currently focused on pouch-shaped batteries, which use an aluminum laminate sheet as a sheathing member, due to advantages such as light weight, low manufacturing cost, and easy shape change.
Recently, a pouch-shaped battery structured such that a stacked or stacked/folded electrode assembly is mounted in a pouch-shaped battery case formed of an aluminum laminate sheet has also attracted considerable attention and use thereof has gradually increased due to advantages such as low manufacturing cost, light weight, and easy shape change.
Electrode tabs of such a pouch-shaped secondary battery are generally formed of a foil having a thickness of 0.5 mm or less. Plural electrode tabs (i.e., cathode or anode tabs) are coupled to an electrode lead (i.e., a cathode or anode lead) through ultrasonic welding to form an electrode terminal. In a state in which such electrode terminals are arranged in a longitudinal direction such that the electrode terminals are successively adjacent to each other, the electrode terminals are coupled to each other through welding. Then, the arrangement of battery cells is folded into pairs of overlapping battery cells to manufacture pouch-shaped secondary batteries.
Electrode terminals of the battery cells are connected to each other or electrode terminals and bus bars of the battery cells are connected to each other through laser welding or ultrasonic welding.
However, in the above structure, bus bars are fixed to a battery module through repetitive welding processes. Therefore, when impact or vibration is applied to the battery module, causing relative movement of the battery cells and the battery module, external force is focused on electrode terminals through which bus bars and battery cells are connected to each other or electrode terminals through which battery cells are connected to each other, thereby causing structural weakness.
The battery module also suffers from such a problem when the battery module is mounted to a vehicle or the like such that the battery module is repetitively exposed to external impact and vibration.
In addition, cathode terminals are much more vulnerable to external impact and vibration than anode terminals due to inherent weakness of the material of cathode terminals formed of aluminum as compared to anode terminals formed of copper.
Thus, there is a great need to provide a secondary battery that fundamentally prevents internal short circuit and breakage due to external impact while providing large capacity and high power and maintaining structural reliability.